Expandable styrene polymer particles containing 1 to 20 weight % of a volatilizable expanding agent such as propane, butane, pentane, methyl chloride or dichlorofluoromethane in styrene polymer particles are already known. These expandable styrene polymer particles produce pre-expanded cellular polymer particles by heating them at temperatures above the softening point of the polymer particles. Cellular shaped articles are obtained from the above pre-expanded polymer particles by heating, for example, with steam, at a temperature above the softening point of the polymer particles in a mold and fusing the polymer particles. Therefore, almost all cellular shaped articles are produced by using polymer particles, at the present time.
The distribution of cells in the cellular shaped articles depend upon the cellular structure of the pre-expanded polymer particles. The distribution of cells in the pre-expanded polymer particles have an influence, not only on the quality of the resulting cellular shaped articles, but also on the operation of preparing the cellular shaped articles. Generally, preexpanded polymer particles are heated by steam and fused in a mold to produce articles having the shape of the mold. However, the cellular shaped articles have a tendency to deform, if the articles are removed without cooling to a temperature under the softening point of the polymer particles. In order to remove the cellular shaped articles without changing the shape, it is, generally, necessary to cool for a longer period of time than the heating time. Therefore, the cooling time is a very important factor in the manufacturing cycle of the cellular shaped articles. After molding, cellular shaped articles are cooled in the mold by a suitable cooling medium, for example, by water. Cooling must be continued until the internal portions of the article have cooled to a temperature below the softening point of the polymer. If it is removed from the mold beforehand, deformation may result. Since cellular shaped articles are good insulators, relatively long periods are necessary to cool the moldings. The time period after which the cellular shaped articles may be removed from the mold without any subsequent deformation is usually called the "minimum residence time".
For example, the minimum residence time of cellular shaped articles having fine cells may be shortened in comparison with those having large cells, since the membranes of the cells become thinner as the cells becomes smaller and the remaining expanding agent is released more rapidly. The produced cellular shaped articles having fine cells do not deform, even if the articles are removed from the mold while the articles are hot, since the inner pressure of the articles and post-expanding properties have been minimized. It becomes possible, therefore, to substantially shorten the period for cooling cellular shaped articles and, moreover, to obtain cellular shaped articles having highly desirable properties, as observed on a cut surface, due to the presence of very fine cell sizes.
It is difficult to control the cell size of pre-expanded polymer particles, since the distribution of the cell size is influenced by many factors, such as kind and/or quantity of expanding agent, the degree of polymerization of the polymer, e.g. styrene polymer and other unknown factors.
After investigating a process for obtaining preexpanded polymer particles having fine cells, we have found the following facts.
Pre-expanded styrene polymer particles expanded to a volume of 70 times the original size of the expandable styrene polymer particles, have a cell size in the range of from 0.01 to 0.3 mm in a diameter as observed on a cut surface. Cell sizes of from 0.1 to 0.3 mm are considered to be large cells. Cellular shaped articles having large cells are hard and have good heatresistant properties. However, long cooling periods are required for cooling the cellular shaped articles in the mold. If such cellular shaped articles are removed from the mold before sufficient cooling, the articles further expand, because release of the expanding agent remaining in the cells has been delayed due to the thick membranes of the cell. Moreover, the cut surfaces or shaved faces of the resulting cellular shaped articles are coarse, due to the large cell size.
On the other hand, a cell size of smaller than 0.1 mm in diameter are considered to be fine cells. Cellular shaped articles having fine cells provide the advantage of shorter cooling periods, since the inner pressure of cells drops quickly, and, in turn, provides cut surfaces or shaved faces of the articles having desirable fine properties. However, the surfaces of the pre-expanded particles are glossy and the surfaces of cellular shaped articles made therefrom are partially melted. Therefore, articles of inferior quality are produced.
In order to obtain pre-expanded styrene polymer particles having fine cell size, U.S. Pat. No. 3,565,835 (Heinz Weber et al.) shows the use of ammonium sulfate and U.S. Pat. No. 3,503,905 (Ludwig Zuern et al.) shows the use of a brominated polymer of a 1,3-diene. These compounds are effective to obtain cellular shaped articles having fine cell size, however, they also have some defects. For example, it is difficult to obtain homogeneous fine cell size, since the polymer particles are influenced by storage temperatures of the expandable polymer particles and the heating temperature during the course of the pre-expansion step. Moreover, the obtained cellular shaped articles have a tendency to form an undesirable yellow color upon exposure to sunlight, ultraviolet rays etc. for long period of time, since the above mentioned compounds remaining in the articles are influenced by, for example, ultraviolet rays etc.